This specification relates to electrical connectors which have to carry very high powers in compact spaces in hostile environments and covers both the connector design and method of assembling it in the field.
The oil production industry has to contend with some of the most inhospitable conditions anywhere. These include wide temperature variations, e.g. −40° C. (storage of equipment in Alaska) to +120° C. (downhole), thermal shock, high pressures and highly corrosive, abrasive environments. A further, and frequently major, factor is mechanical shock and fatigue due, particularly, to vibration caused by fluid flow past the connector. A typical installation is shown in FIG. 1 where wellhead 1 is shown on seabed 2. Hole liner 4 is suspended from tubing hangar 3 and supports a number of items, e.g. electrical submersible pump (ESP) 5, which is powered via protected cable 6. A key item is the motor lead extension (MSE) connector 7, which is sometimes known as a ‘pothead’.
This specification deals particularly with connectors 7 such as this, which are inaccessible once in position and have to last, at least, for the lifetime of pump 5.
Previous experience with currently available connectors 7 has been unsatisfactory as the factory made units often suffer from problems relating to temperature variations, and the resultant mechanical stresses generated and electrical failure due to movement of the contacts under thermal, or operating pressure, effects. One particular problem is vibration, induced by the high flow rate of oil and/or gas in liner 4. Another factor is that cable 6 must be cut to the exact length so that there is a minimum of free cable as slackness can result in cable damage due to fretting and/or impacts as the cable whips around in liner 4. High frequency vibrations, even though of only small amplitude, can, over a period of time, have a significantly damaging effect on a cable, e.g. fatigue, chafing of insulation, etc.
Cable 6 is attached to connector 7 and it is this which effectively has to provide the ‘reaction’ to these vibrations. Thus, the cores inside connector 7 are subject to high frequency, cyclical, axial and bending forces. The effect of these forces is to weaken the connection both mechanically and electrically and, usually, lead to premature electrical failure, with consequent serious loss of production.
As explained, the connector provides the ‘mechanical reaction’, i.e. acting in a pin-jointed or encastrè capacity. Some current connectors use a multiple metal-rubber disc compressed sandwich to form the seal. This can leak due to the effects of thermal cycling and set when the rubber does not fully recover its previous size on cooling (in operating practice, it is common to have to shut down the well, either for downhole maintenance or for work on the seabed or surface equipment; this allows cycling from 100+° to 4° C. {sealed temperature} and back.) Here the reaction point is where the conductor enters the crimped or soldered joint. In others, moulded rubber is used and here the reaction point is where the insulated core enters the rubber insulator. Clearly, there is a need to spread the reaction over as long an axial length as possible to minimise fatigue effects.
Current practice is for cables 6 and connectors 7 to be factory assembled in fixed lengths, often 16.76 m (55 ft). Though means-to shorten cables, e.g. by coiling, etc., are known, space is extremely limited in liner 4 and wellhead 1. Furthermore, cable 6 is armoured and not easily bent. Additionally sharp bends place unnecessary stresses in the cable.
There is thus a need for a high powered, precision-made electrical connector which can be assembled on site, quickly and reliably and to exact lengths, preferably by semi-skilled personnel. Preferably, such connectors should be able to accommodate external forces placed on them without deterioration.